Multiple channel receivers are capable of receiving signals over various bands of the frequency spectrum. When an incoming signal having a known frequency is to be received, a receiver that is tuned to the particular channel corresponding to the signal's frequency can receive the signal. A difficulty arises, however, when the signal to be received is of an unknown frequency. In such a case, the receiver must determine the incoming signal's frequency in order to then tune to the appropriate channel and receive the signal. Unfortunately, it is difficult to determine the specific frequency channel which yields the best signal quality. This difficulty occurs because signals may be detected in more than one channel due to imperfect out-of-band rejection of the channels adjacent to the one yielding the best reception quality, or the application of pulse modulated signals to the receiver which are prone to cause detections in different channels. Consequently, the receiver may be tuned to an inappropriate adjacent channel, resulting in imperfect signal reception, signal distortions, and general delays in data and/or voice reception. Thus, it is important to have a device which could determine the specific frequency channel over which a signal exhibits its largest amplitude. With this capability, a channelized receiver tuned to this specific frequency channel will receive the signal with minimized distortion.
It is known in the art to identify the frequency of an RF signal in a channelized receiver. An example of one patent that discloses such technique is U.S. Pat. No. 4,301,454, issued Nov. 17, 1981, entitled "Channelized Receiver System" by D. E. Bailey. The Bailey patent discloses a receiver which identifies the frequency channel in which an unknown signal is received by comparing the amplitude of a filtered signal having a largest amplitude with that of a filtered signal having a next largest amplitude. The receiver splits a received RF signal into at least two portions and applies each portion to a bank of filters. Each bank is comprised of filters defining alternate portions of a search bandwidth. Within each filter bank, the signal portions are applied to individual filters via circulators. When the frequency band of a particular filter corresponds to the signal's frequency, the signal is bandpassed through the filter. Where the signal frequency does not correspond to the filter frequency band, the signal rebounds back to the circulator which directs the signal to the next filter, where this process is repeated and continued.
The signal portions which pass through each filter bank are then applied to a voting logic device. The voting logic device determines the specific filters in each group having the largest amplitude output signals and compares the amplitude relationship that exists between these determined signals. When the filters having the largest output signals have pass bands defining contiguous portions of the search bandwidth, the signals are used by a computer preprogrammed with each of the filter pass band characteristics to calculate the RF signal frequency. The computer, which makes the calculations by determining the difference between the output signals, then generates a tuning signal to be used to tune a narrowband receiver to an appropriate frequency channel.
The teaching of Bailey is exemplary of the complexity of conventional approaches for determining the frequency of a received RF signal.
It is one object of this invention to provide for a simplified system of small physical size which quickly and accurately determines the frequency of a received RF signal for use in a channelized receiver, and that is suitable for receiving both continuous wave and intermittent (e.g., pulsed) transmission signals.